An inductor is an electronic component designed especially to provide a controlled amount of inductance, which is a quantitative measure of the ability of a device to store energy in the form of a magnetic field. Inductance is conventionally represented by the capital letter L, and the unit of inductance is henry. One henry is the amount of inductance to generate 1 volt of voltage with a current that changes at the rate of 1 ampere per second. Conventionally, the inductors generally consist of a length of wire wound into a solenoidal or toroidal shape. An inductor is a key element in a wide variety of radio frequency (RF) circuits, such as those used in cellular telephones, wireless modems, and other types of communication equipment.
With the fervent drive for continuous miniaturization of electronic circuits especially with regard to consumer goods, many digital and analog circuits have been successfully implemented into silicon or semiconductor based integrated circuits (ICs). And the trend is increasing at an even faster rate. The goal to achieve miniaturization involves not only the task to achieve compactness in terms of mechanical packaging, but also the task to decrease the cost of manufacturing. One area that remains a challenge to the goal of miniaturization is the manufacturing of radio frequency (RF) circuits.
Silicon technologies have been applied to provide integratable inductive structures. However, one of the problems in applying silicon technologies to RF applications is the relatively low quality factor, or the so-called Q values, of the inductors fabricated from silicon technology. The quality factor Q is related to the inductance L and resistance R of the inductor according to the following equation (.omega..sub.0 is 2.pi. times the frequency): EQU Q=.omega..sub.0 *L/R
Planar inductors with a spiral structure are the most common type of inductors within integrated circuits. FIG. 7 shows an example of such spiral structured silicon-based inductor. The overall inductance L of the conductive path of the silicon based inductor is calculated by summing the inductance of each sub-length of each individual segment. During operation, fields are created by current flowing through the spiral pattern which tend to cause the current to flow along the inner or shorter edges, i.e., the paths of least resistance. Such a current flow can cause a significant increase in resistance with increasing frequency. With the need for further miniaturization, there is a need to pack more segments into a smaller area, and the current flow between segments becomes even more significant.
U.S. Pat. No. 5,227,659, the content thereof is incorporated by reference, discloses a method by which standard CMOS technology is used to fabricate a multiturn coil having its axis normal to the layers of oxide. For a coil 100 microns on a side, an inductance on the order of 10 nH can be achieved. This value can be increased to as high as 0.1 mH by including a magnetic core in the design.
U.S. Pat. No. 5,446,311, the content thereof is incorporated by reference, discloses a method for forming a monolithic high-Q inductor structure having multiple metalization levels using a conventional integrated circuit technology. In this monolithic high-Q inductor structure, inductor turns utilize these multiple levels to reduce the inductor resistance. The inventors of the '311 patent claimed that inductors with Q values above five can be integrated with this approach at radio and microwave frequencies.
U.S. Pat. No. 5,545,916, the content thereof is incorporated by reference, discloses a high-frequency inductive structure integratable with a semiconductor integrated circuit. In this invention, a conductive path forming the structure is arranged so extra conductive material is located at portions of the cross-section of the conductive path where current tends to flow at high frequency. In the '916 invention, the location at which the extra conductive material is disposed is critical.
U.S. Pat. No. 5,635,892, the content thereof is incorporated by reference, discloses a high-Q inductive structure with increased self-inductance and improved Q at high frequencies. The inductive structure disclosed in the '892 patent contains (1) an electrical conductor providing a conductive path formed as a spiral planar pattern upon a substrate, and (2) a core of magnetic material in proximity to and facing the planar pattern which defines an opening in the central region of the planar pattern.
While the above-mentioned inventions have their merits, they all involve relatively complicated fabrication procedure. Moreover, with the significant market potential of silicon-based inductors, it is highly desirable to devote research and development efforts so that improved high-Q inductive structures can be discovered and implemented into the currently existing integrated circuitry with minimum device size.